A mixture of water and salt is a homogeneous solution, meaning the salt is dissolved completely throughout the water. Separating these two components is a physical process that relies entirely on the significant differences in their physical properties. Water is highly volatile with a low boiling point of 100 degrees Celsius, making it easy to change into a gas. Conversely, salt (sodium chloride) is non-volatile and possesses an extremely high boiling point (around 1,413 degrees Celsius). This difference in volatility is the scientific principle that allows various separation methods to be effective.
Simple Evaporation to Recover Salt
Simple evaporation is the most straightforward method to separate the mixture, primarily aiming for the recovery of the solid salt. This technique uses a heat source to accelerate the phase change of the solvent, water, leaving the non-volatile salt behind. The salt solution is heated in an open container, causing water molecules to gain kinetic energy and escape as steam or water vapor into the atmosphere.
As the water evaporates, the remaining solution becomes progressively more concentrated until it reaches its saturation point. Continued heating causes the remaining water to boil away, and the dissolved salt precipitates out, forming solid crystals. This process is used industrially in salt flats, where solar energy slowly evaporates seawater, leaving large deposits of sea salt. In a laboratory setting, this separation is achieved faster by boiling the water over a burner or stove.
Distillation for Recovering Pure Water
When the goal is to recover the pure liquid component, water, a controlled process called distillation is employed. Distillation is a two-step process: controlled evaporation followed by condensation. The salt water is heated in a sealed flask, causing the water to vaporize into steam.
The key difference is that this steam is directed away from the boiling flask into a separate apparatus, typically a condenser. Inside the condenser, the hot water vapor flows through a tube that is continuously cooled by circulating cold water. This rapid cooling removes heat from the steam, causing it to condense and revert to liquid water, which is collected in a receiving flask.
This collected liquid, known as the distillate or distilled water, is nearly pure. The non-volatile salt remains trapped in the original boiling flask because its boiling point is too high to vaporize. The process is widely used to produce high-purity water for scientific and industrial applications.
Membrane Separation Techniques
An alternative approach that does not rely on a phase change like boiling is the use of membrane separation techniques, most commonly Reverse Osmosis (RO). This method exploits the difference in the physical size between water molecules and dissolved salt ions. Reverse osmosis involves applying significant pressure to the salt solution, forcing it against a semi-permeable membrane.
The semi-permeable membrane contains pores fine enough to allow small water molecules to pass through while physically blocking the larger dissolved salt ions. This applied pressure must be greater than the natural osmotic pressure to reverse the flow of water, pushing it from the concentrated salt side to the pure water side. Reverse osmosis is a highly effective, continuous process used primarily for large-scale industrial desalination, converting seawater into potable drinking water.